The characteristics of a cochlear implant's front end play an important role in the sound quality (and hence speech recognition or music appreciation) experienced by the cochlear implant (CI) user. These characteristics are governed by the components of the front-end including a microphone and an A/D converter in addition to the acoustical effects resulting from the placement of the CI microphone on the user's head. The acoustic characteristics are unique to the CI user's anatomy and the placement of the CI microphone on his or her ear. Specifically, the unique shaping of the user's ears and head geometry can result in substantial shaping of the acoustic waveform picked up by the microphone. Because this shaping is unique to the user and his/her microphone placement, it typically cannot be compensated for with a generalized solution.
The component characteristics of the microphone must meet pre-defined standards, and this issue can be even more critical in beamforming applications where signals from two or more microphones are combined to achieve desired directivity. It is critical for the microphones in these applications to have matched responses. Differences in the microphone responses due to placement on the patient's head can make this challenging.
Beamforming is an effective tool for focusing on the desired sound in a noisy environment. The interference of noise and undesirable sound tends to be very disturbing for speech recognition in everyday conditions, especially for hearing-impaired listeners. This is due to reduced hearing ability that lead, for example, to increased masking effects of the target signal speech.
A number of techniques based on single and multiple microphone systems have already been applied to suppress unwanted background noise. Single microphone techniques generally perform poorly when the frequency spectra of the desired and the interfering sounds are similar, and when the spectrum of the interfering sound varies rapidly. By using more than one microphone, sounds can be sampled spatially and the direction of arrival can be used for discriminating desired from undesired signals. In this way it is possible to suppress stationary and non-stationary noise sources independently of their spectra. An application for hearing aids requires a noise reduction approach with a microphone array that is small enough to fit into a Behind The Ear (BTE) device. As BTEs are limited in size and computing power, only directional microphones are currently used to reduce the effects of interfering noise sources.